1. Field of the Invention
The present invention relates to a conductive adhesive formed by mixing conductive fillers into resin. The conductive filler can be made by providing silver particles on a core material made of copper-based metal (copper or an alloy including copper). The conductive adhesive can be suitably used for bonding two members.
2. Description of Related Arts
Generally, a conductive adhesive is a mixture of resin for performing a bonding and conductive fillers for providing an electrical conductivity. Specifically, the conductive adhesive is formed by mixing the conductive fillers made of copper and/or silver into thermosetting resin, e.g., epoxy resin. For example, the conductive adhesive is used for an electrical connection between an electronic part and a substrate.
JP-A-2003-68140 and JP-A-2001-43729 disclose conductive adhesives, in which conductive fillers formed by covering a surface of a core material made of copper-based metal with particles (e.g., powder) made of silver are used.
After the conductive adhesive is coated on an electrode of a connecting portion, the resin in the conductive adhesive is heated so as to be hardened. The resin is contracted due to the hardening so that the conductive fillers contact with each other, and the conductive filler and the electrode contact with each other, thereby an electrical conduction can be performed.
In the conductive adhesive using the conductive filler, a specific surface area of the conductive fillers can be large due to the silver particles. Moreover, the conductive adhesive can be sintered at a lower temperature due to the silver particles, a melting point of which is relatively low.
Accordingly, a metallic junction, i.e., metallic bonding, of the silver particles can be formed as a filler connection, thereby a reliability of the filler connection is higher than that of a contact conduction.
In the conventional conductive adhesive using the conductive filler formed by covering the surface of the core material made of the copper-based metal with the particles made of the silver, a potential difference between the filler and a tin (Sn) electrode used for the electronic part is made to be low by using the copper-based metal for the core material. Thus, a galvanic corrosion can be prevented.
However, because the surface of the core material is coated with the silver particles, the surface of the core material made of copper is easy to be exposed from a clearance between silver particles. Therefore, the exposed part of the copper is easy to be oxidized.
Further, when a bonding is performed by using the conductive adhesive, in which the copper is oxidized, the metallic junction of the fillers by the silver particles is difficult to be formed due to the copper oxide film on the fillers.
Accordingly, after the bonding, the junction between fillers becomes inadequate. Thus, an electrical resistance of the conductive adhesive may be increased. Further, when the copper oxidation proceeds by heat, the resistance may be more increased.